Override an inner function of a prototype function in javascript

Question

I have a javascript Object like below:

var Loader = function() {}
Loader.prototype = {
    constructor: Loader,
    foo: function(a, b) {
        function bar() {
            return a + b;
        }
        return bar();
    }
};

I need to override the bar() function present inside foo . The foo function can be overridden as:

Loader.prototype.foo = function() {}

I have tried the same for inner function but no luck.

Loader.prototype.foo.bar = function() {}//doesnt work
Loader.prototype.foo.protoype.bar = function() {}//doesnt work

I would be very happy if someone could point me in the right direction. Thanks in advance

You cannot override the inner function because it's scope is restricted to the function foo. — Golak Sarangi
– Golak Sarangi, Commented May 5, 2016 at 5:45
I guess you will have to override (copy paste) the entire foo() function and only change the relevant parts. Yes, it can be bad if updates arrive for what you are overriding (a 3rd party lib, perhaps). — Thernys
– Thernys, Commented May 5, 2016 at 5:50

Dmitri Pavlutin · Accepted Answer · 2016-05-05 06:10:31Z

3

In the current declaration, the bar() function cannot be modified. It exists in the scope of foo() method and cannot be changed from outside.

An alternative solution for a dynamic override could be:

var Loader = function() {}
Loader.prototype = {
    constructor: Loader,
    foo: function(a, b, barFun) {
        if (typeof barFun !== 'function') {
           barFun = function() {
              return a + b;
           }
        }
        return barFun.call(this, a, b);
    }
};

When calling foo(), it's possible to use a custom function as the 3rd parameter.
Then you can use it:

var loader1 = new Loader();

loader1.foo(1, 2); // prints 1 + 2 = 3, default bar() function

loader1.foo(1, 2, function(a, b) {
  return a - b;
}); // prints 1 - 2 = -1, custom bar() function

Check the working demo.

edited May 5, 2016 at 6:10

answered May 5, 2016 at 5:55

Dmitri Pavlutin

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6 Comments

Sandeep Over a year ago

Thanks for the quick answer, looks like i will have to go with this approach.

jfriend00 Over a year ago

But bar can effectively be replaced by just assigning a different function to foo that uses different code than what bar is doing and this can be done from the outside.

Sandeep Over a year ago

@jfriend00 which means if I have 60 different functions inside foo() i will have to override them all.

Dmitri Pavlutin Over a year ago

@jfriend00 This is a skeleton example. Before calling bar(), the foo() may contain a lot of logic. Simply overriding it will create duplications.

Sandeep Over a year ago

@jfriend00 I wanted to override buildImage() inside parse() in this : github.com/mrdoob/three.js/blob/master/examples/js/loaders/…. It was much easier to ask with a simpler example.

|

laoqiren · Accepted Answer · 2016-05-05 05:59:12Z

0

In my opinion,the inner bar function is not the method of the outter foo function which is the method of loader object.Hope that it will help you

answered May 5, 2016 at 5:59

laoqiren

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2 Comments

Sandeep Over a year ago

No this doesn't help. bar() is function of foo() which is function of Loader.

laoqiren Over a year ago

I just meant that it's not the method of it.

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